Spastic muscles are in the state of concentric contractions can’t do eccentric lengthening. Spastic muscles are in shortened state and active eccentric contraction in difficult. This can be due to
a. Weakness of antagonistic muscle
b. Due to reciprocal inhibition causes relaxation of antagonistic muscle
Spastic muscles act like spring that work in concentric contraction and remains in contraction mostly (flexed position) which creates motor imbalance between them and antagonist to them. Therefore strengthening antagonist assists in reducing tone of spastic muscles as this allows lengthening of spastic muscle!!
So in the initial phase of neuro rehabilitation of hemiplegic patients we should concentrate more on eccentric and static (placing reaction) contraction.
Avoid strengthening the concentric contraction. Because if the spastic muscle becomes strong then it is difficult to initiate the eccentric contraction.
During training of upper limb control we train the elbow flexion and shoulder flexion. The common mode is to train concentric contraction. We tell the patient to lift upper limb against gravity. This indirectly trains the flexor synergy make it strong then it becomes difficult to break the synergy.
So what to do? Work on placing reaction in upper limb. Place the shoulder in flexion above 90 degree and let him hold it and followed by slow lowering the upper limb (eccentric contraction).
What is the advantage of eccentric contraction? Eccentric contraction helps early and better in recruitments of motor units than concentric contractions. Eccentric contraction can generate more force with less motor unit recruitment. Concentric contraction requires more motor unit recruitment for even generation minimum muscle contraction. So it becomes difficult to initiate concentric contraction than eccentric contraction.
So in the spastic muscles we should concentrate more on eccentric lengthening. Increase in eccentric lengthening of muscles indirectly helps in reducing spasticity.